1. Field of the Invention
The present invention relates to a process for producing a polymeric material open cell foam utilizing the decomposition of a foaming agent. That is, in the present invention, a thermoplastic resin polymer blend system containing a photosensitizing agent and a thermally decomposable foaming agent and containing 1,2-polybutadiene as one component is irradiated with ultraviolet light to cross-link mainly the 1,2-polybutadiene, and then the composition is heated to a temperature higher than the softening point of the 1,2-polybutadiene and the softening point of the thermoplastic resin polymer blended therewith and higher than the decomposition temperature of the foaming agent to form an open cell foam.
A main aspect of the present invention is that it enables an open cell foam of a thermoplastic resin polymer utilizing the decomposition of a foaming agent, which has been believed to be difficult, to be easily produced in a practical manner, and that the resulting open cell foam has many uses based on the physical properties of 1,2-polybutadiene and the other thermoplastic resin polymer blended therewith.
2. Description of the Prior Art
Expanded materials or porous materials can generally be classified as having a closed cell foam structure or an open cell foam structure. With respect to uses, too, these cell structure differences give rise to distinctly different uses. For example, a material with a closed cell foam structure is used as a buoyant material, a heat insulating material, a packing material, and the like, whereas a material with an open cell foam structure is used as a filter material, a sound-absorbing material, and the like. On the other hand, both types of materials are used as a cushioning material.
Polymer material foams can be classified as in the following Table 1 according to the processes for producing closed cell foams and open cell foams.
Table 1 ______________________________________ Classification of Polymer Material Foam Forming Processes Foaming Process Closed Cell Foam Open Cell Foam ______________________________________ Gas-Mixture PVC (Airex process) PVC (Elastomer Process process, etc.), Rubber (soft; Dunlop process) Foaming Agent- Decomposition Process Ordinary- PE (radiation cross- pressure linking process, Foaming chemical cross- Process linking process) PP (radiation cross- linking process) PVC (leather) Rubber (soft; Talalay process) Silicone, Glass Extrusion PE, PP, PVC, ABS, Foaming etc. Process Press Foaming PE, PP, PVC (Kleber Process process) Acryl Resin, Rubber (hard) Injection Various Thermoplastic Foaming Resin Polymers Process Solvent- Vaporization Process Within-Mold PS (Beads process) Foaming Process Extrusion PE, PS, Various Foaming Thermoplastic Resin Process Polymers (Celka process) Two Liquid- Epoxy Resin, Phenol Resin, Mixture Silicone (pyranyl) Urea Resin Process Chemical Reac- Polyurethane (hard) Polyurethane (soft) tion Process Elution Process PE, PVC, Vinylon, Viscose Sintering Inorganic, Process Polyethylene, Nylon, Fluorine-Containing Resin Others Syntactic Carbon ______________________________________ Note: PVC: polyvinyl chloride PE: polyethylene PP: polypropylene ABS: acrylonitrile/butadiene/styrene PS: polystyrene (From Maki et al., Plastic Foam Handbook, published by Nikkan Kogyo (February 28, 1973))
This Table shows that there is absolutely no industrial process for producing an open cell foam utilizing a foaming agent-decomposition process. The present invention provides a novel process in this field which is industrially practical.
The mechanism of the formation of an open cell foam is described below. According to the elution process, a base resin is filled with a soluble material, and then the soluble material is extracted therefrom. This process is suitable for producing porous bodies having fine pores, since the size of the soluble filler becomes the size of resulting cells. The void ratio is decided by the amount of the filler. Use of too much filler would destroy the cells, whereas when too little filler is used the cells would not be connected to each other, resulting in filler remaining and formation of unexpanded products. Therefore, with the elution process, only cellular materials having a low expansion ratio of a comparatively narrow range are obtained. In addition, the extraction requires a long time and, thus, the products become extremely expensive.
The sintering process is a process utilizing, as cells, voids formed upon sintering resin particles. When the viscosity upon sintering is too low, voids are filled and closed with the fluid particles, resulting in a failure to form an open cell material. Therefore, this process can be applied only to resins which are sintered to each other and for which fluidized deformation does not occur. In this case, the ratio of cell volume to cell wall depends upon the size of particles. Since there is a physical limit to the particle size of the resin particles, there is a limit to the void ratio. Thus, this process provides only cellular materials of a comparatively low expansion ratio.
These above two processes can also be practically applied, although slightly, to general-purpose thermoplastic resin polymers of polyethylene and polyvinyl chloride. However, the application of these two processes is not economically advantageous.
The gas-mixture process, the two liquid-mixture process, and the chemical reaction process are essentially processes of adjusting the relationship between the viscosity of a resin and the gas pressure. Of these, the gas-mixture process is a process comprising mechanically mixing an inert gas into a surface active agent-containing slurry resin at a low temperature and under high pressure by stirring at high speed to thereby disperse and absorb the gas in the slurry, and then allowing the resin to foam due to the expansion of the gas. This process can be used in producing an open cell foam from a paste-like polyvinyl chloride and rubber latex. This process is applicable only where a resin slurry can be prepared.
In the two liquid-mixture process as an example of the solvent-vaporization process, a volatile foaming agent is added to a liquid resin and, under stirring at high speed, a hardening agent is added. Thus, the foaming agent is vaporized away due to the reaction heat of hardening, thus forming an expanded material. The cells are finally stabilized by the solidification of the resin. This process can be used with hardenable resins such as phenol resins and urea resins.
Further, the chemical reaction process is represented by the process for producing a polyurethane foam, and is a process of incorporating in gas produced simultaneously with the polymer-forming reaction.
As is described above, suitable processes for producing an open cell foam are inevitably determined by and dependent upon the characteristics of the resins used. However, processes for producing an open cell foam utilizing a foaming agent, which are believed to be extremely advantageous industrially, have not so far been industrially put into practice. In particular, establishment of a process for producing an open cell foam of general-purpose thermoplastic resin polymers utilizing a foaming agent-decomposition process has eagerly been desired.
In general, the foaming agent-decomposition process and the solvent-vaporization process are mainly employed for expanding thermoplastic resin polymers. However, it is difficult to industrially produce an open cell foam of good quality having sufficient commercial value utilizing these processes, although a closed cell foam can easily be obtained.
In general, the mechanism of the formation of an open cell foam using the gas process can be roughly classified into three types: (1) the type wherein materials have an aggregating property such as a urethane and, a solid polyurethane is forced by a gas generated during polymerization to surround a cell and form a cell wall; (2) the type wherein elongation of a polymer material as a solid upon expansion is small, and cells associate with each other to form an open cell foam; and (3) the type wherein cells in an essentially closed cell foam are partly broken when the cells are further expanded to such a degree that the cell wall can no longer resist the gas expansion pressure, thus forming a partly open cell foam. The diffusion coefficient of the gas, the manner of mechanical stirring, the stage and degree of pressure application, and the like greatly influence the cell formation mechanism. In any way, in order to obtain an open cell foam by utilizing the expansion pressure of gas, the system must be solidified to a degree such that, even when the cells are expanded without breaking the cell nuclei to connect the cells together, the cells will not be destroyed and contract.
A sharp reduction in viscosity of thermoplastic resins occurs at a temperature higher than the melting point of the thermoplastic resin. Experimentally it has been shown that the viscosity of a thermoplastic resin polymer is too low to maintain its shape as a molding when a gas is present in a volume about 5 times larger than that of the thermoplastic resin polymer. When the viscosity of the resin forming the cell wall is not sufficient to resist the expansion pressure, the cell wall will be broken, resulting in a contraction of the cells and, in an extreme case, a foam with a low expansion ratio or unexpanded products will be obtained. Therefore, a process in which the viscosity of the resin is increased by cross-linking is employed. The so-called pre-cross-linking procedure preceding foaming and expansion generally provides a high closed cell ratio. A simultaneous cross-linking reduces the gas efficiency, and tends to result in uneven cells. This also provides a high closed cell ratio. A post-cross-linking is useless since the viscosity of the resin is increased after the gas has escaped. Formation of partly open cells by adjusting the degree of pre-cross-linking or the stage and degree of simultaneous cross-linking under application of pressure or under ordinary pressure has been attempted on a laboratory scale. However, the results do not have good reproducibility, and products of good quality have not yet been obtained. In addition, these have been conducted batchwise, and complicated procedures are involved. Thus, productivity is not achieved. These are the great difficulties which have inhibited the practice of the processes on an industrial scale.